Such a device constitutes, in particular, an outer or inner connection termination under dielectric gas pressure for the high tension cable of an apparatus such as a transformer or an overhead line, for example. It is more particularly applicable to the ends of a cable having synthetic insulation under a tension of tens of thousands of volts, of the order of 100 kV to 400 kV, from which discharge currents in the event of a failure may reach tens of thousands of amps.
In conventional manner, such a device comprises an insulator surrounding the end of the cable and defining a sealed enclosure thereabout under pressure of a dielectric gas. The insulator is closed in gastight manner at a "top" end thereof by a metal cap which passes through the cable conductor to connect it to a terminal fixed on said cap. Its opposite "bottom" end is also closed relative to the cable by means of a metal part.
Such a protection device is described, in particular, in Document FR-A-2 590 739 relating to a high tension cable having synthetic inner insulation and a central conductor which is bared immediately upstream from the closure cap of the insulator for connection to the terminal.
In such protection devices, the enclosure under dielectric gas pressure is designed to retain an operating pressure of about 8 bars to 10 bars and its volume is about 50 liters or more. A short circuit due to a fault in the inner insulation of the cable and/or to excessive surges may cause the metal portions and the insulating portions at the end of the cable to melt, a sudden increase in dielectric gas pressure, discharge currents through the dielectric gas, and an explosion of the insulator, with metal debris, insulator debris, and all of the decomposition products of the adjacent materials being projected over a distance.
Until now, in order to provide protection against possible explosion of the insulator, it has been the practice for the wall of the enclosure to be provided beyond the bottom of the insulator with a rupture disk that enables the dielectric gas to escape to the atmosphere together with the metal debris and all of the decomposition products of the adjacent materials entrained thereby. However, it has not always been possible to prevent metal debris impacting against the inside surface of the insulator, thereby damaging or breaking it. In addition, the arc is not always extinguished by the dielectric gas as quickly as could be desired.
An object of the present invention is to provide a protection device that is more effective to avoid an explosion of the insulator and any impact of metal debris against the inside surface of the insulator, to ensure that any such debris is entrained towards an orifice that is disengaged by the above-mentioned rupture disk, and to reduce the time required by the dielectric gas to extinguish the arc.